Environmental control system for precision airborne payloads

ABSTRACT

Outside air enters a ram air scoop on an aircraft frame, and is ducted to a ram air control valve. The air control valve outputs a desired air mass flow to a cyclone air-water separator which removes moisture and produces a dry air flow. A heater assembly heats the dry air to a desired temperature and directs the heated air into an equipment bay enclosure on the aircraft. The relative humidity of the heated air is sensed by an air moisture sensor which produces a corresponding signal. Other sensors disposed near a payload in the bay enclosure produce corresponding air temperature and air pressure signals. All the sensor signals are input to a processor or controller configured to activate the air control valve and the heater assembly according to set points for temperature and humidity that are specified for the payload in the bay enclosure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to environmental control systems, andparticularly to systems that provide controlled temperature and humidityenvironments for airborne payloads.

2. Discussion of the Known Art

Airborne reconnaissance, surveillance, and electronics payloads aretypically mounted in an unpressurized equipment bay on an aircraft, andare expected to operate reliably at altitudes from sea level to 60,000feet or higher above mean sea level. Such payloads often includeprecision optics, focal plane arrays, and electronics. The electronicequipment may include recorders, data links, sensor control units,control electronics units, and countermeasures systems.

The performance of optical system components is affected greatly bythermal expansion and contraction. Additionally, opto-mechanical andelectronic components that are integrated within precision opticalsystems are also sensitive to temperature and moisture. Thus, in orderfor precision optical system payloads to operate properly and accuratelyover a given aircraft mission flight profile, the temperature and thehumidity inside the equipment bay must be controlled according to thespecifications imposed by the system payloads. The temperaturerequirements are typically 25 deg. C.+/−2 deg. C., without condensationdeveloping inside the bay throughout the mission flight profile.Depending on the application, the internal equipment bay temperaturespecification can range between −30 deg. C. to 40 deg. C., with a settemperature within this band controlled to a tolerance of +/−5 deg. C.to +/−1 deg. C. throughout the flight profile.

At present, either an air cycle machine (ACM) or a vapor cycle machine(VCM) and associated fans are used for thermal control of electronicsand imaging systems aboard aircraft. Airborne ACM systems are disclosed,for example, in U.S. Pat. No. 6,058,715 (May 9, 2000) and No. 6,427,471(Aug. 6, 2002), and in U.S. Patent Application Pubs. No. 2006/0059942(Mar. 23, 2006) and No. 2008/0022688 (Jan. 31, 2008). Airborne VCMsystems are described, e.g., in U.S. Pat. No. 5,918,472 (Jul. 6, 1999),and in U.S. Patent Application Pub. No. 2008/0199326 (Aug. 21, 2008).Disadvantages of either system are that the ACM and the VCM machineseach weigh approximately 100 pounds and consume a large amount ofelectrical power and critical volume aboard an aircraft. Also, the costof either system typically exceeds $100,000, with amean-time-between-failures (MTBF) of only about 4,000 flight-hours.

Cyclone separators that remove dust, liquid, and other matter from asupply of air are generally known. Cyclone separators operate bydirecting the supplied air to flow in a conical path, thereby causingmatter carried or entrained within the air to separate from the air bycentrifugal force. A cyclone separator for dehumidifying compressed airfor marine control equipment is disclosed in U.S. Pat. No. 6,019,822(Feb. 1, 2000) which is incorporated by reference. See also, U.S. Pat.No. 6,851,459 (Feb. 8, 2005); No. 7,381,235 (Jun. 3, 2008); and No.7,594,941 (Sep. 29, 2009), all of which are also incorporated byreference. As far as is known, however, cyclone separators have not beenused in systems for controlling the thermal environment of sensitiveairborne payloads.

Accordingly, there is a need for a reliable and light weightenvironmental control system for sensitive airborne payloads,particularly a system that consumes minimal space and electrical poweraboard an aircraft, and which can be constructed and maintained atrelatively low cost.

SUMMARY OF THE INVENTION

According to the invention, an environmental control system for apayload equipment bay enclosure on an aircraft includes a ram air scooparranged with an opening in the forward direction of the aircraft forobtaining a supply of outside air. An air control valve receives theoutside air supply from the scoop and provides a determined air massflow in response to an air control valve signal. A cyclone air-waterseparator has an inlet for receiving the air mass flow from the aircontrol valve, and the separator operates to remove moisture and toprovide a relatively dry air mass flow at an outlet of the separator.

An air heater assembly has a heater intake in communication with theoutlet of the separator for heating the relatively dry air mass flow toa determined temperature in response to a heater signal, and fordirecting the temperature controlled air mass flow toward the equipmentbay enclosure. Prior to entering the enclosure, the relatively dry airpreferably passes through a regenerative desiccant to remove further anyhumidity entrained in the air flow. An air moisture sensor and atemperature sensor are each disposed in a path of the temperaturecontrolled air mass flow from the heater assembly, for producing signalscorresponding to relative humidity and temperature of the air mass flow.One or more air temperature sensors are also disposed in the bayenclosure in the region of a payload when mounted in the enclosure forproducing corresponding air temperature signals, and an air pressuresensor is disposed in the enclosure to produce a signal corresponding toaltitude of the payload.

A processor or controller has inputs coupled to the air temperaturesensors and to the air pressure sensors. The controller is configured tosupply the air control valve signal and the heater signal according tocertain humidity and temperature set points required for the payload inthe equipment bay enclosure. Accordingly, the inventive control systemenables reliable temperature and humidity control in unpressurizedequipment bays at altitudes of up to 60,000 feet or higher.

For a better understanding of the invention, reference is made to thefollowing description taken in conjunction with the accompanying drawingand the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a diagram of an environmental control system according to theinvention;

FIG. 2 is a schematic diagram of the inventive control system includinga controller shown in FIG. 1: and

FIG. 3 is a table showing a controlled state of certain components inthe system of FIG. 1 over part of a typical aircraft mission flightprofile.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram of an environmental control system 10 according tothe invention. The system 10 maintains the temperature and relativehumidity environment necessary for reliable and accurate operation ofsensitive reconnaissance, surveillance, and electronics payloads on highperformance aircraft, unmanned aerial vehicles (UAVs), blimps, and otherplatforms used for reconnaissance, surveillance or monitoring. As usedherein, the term “aircraft” includes all such vehicles and platforms.

In the illustrated embodiment, the system 10 includes a ram air scoop 12that is fixed on an aircraft frame 14 with an open front end 16 of thescoop facing in a forward direction of the aircraft. During flight,outside air that enters the open end 16 of the scoop is directed throughan intake duct 18 to be conditioned and supplied to the interior of anequipment bay enclosure 20 on the aircraft. The bay enclosure 20 ispreferably thermally insulated from the surrounding environment in theaircraft by a shroud 22 of fiberglass or equivalent material having lowthermal conductivity. A window 24 formed in the aircraft frame 14 allowsoutside light or other radiation to enter the equipment bay enclosure 20for imaging or other processing by a payload 26 when mounted inside theenclosure 20. Additional insulation 28 is preferably disposed around thewindow 24 between the aircraft frame 14 and the bay enclosure 20. Thepayload 26 may include, for example, sensitive reconnaissance,surveillance, and/or electronics systems whose reliability and accuracywould be compromised if the air temperature or humidity inside theequipment bay enclosure 20 deviates from specified limits. The payload26 may, or may not, have its own separate enclosure.

A ram air control valve 30 has an inlet for receiving the outside airsupplied through the duct 18 from the ram air scoop 16. The controlvalve 30 operates to provide a determined air mass flow at an outlet ofthe valve in response to a signal from a controller 32 throughout theaircraft's Mach number and altitude, coupled with transient aerodynamicheating from the aircraft power flight condition and the payload's ownheat dissipation which depends on its transient modes-of-operation. Asuitable ram air control valve is described, e.g., in InternationalPatent Application PCT/EP2006/010289 titled Emergency Ram Air InletValve of an Aircraft, published May 18, 2007, as WO/2007/054206 andwhich is incorporated herein by reference.

The controlled air mass flow from the outlet of the control valve 30 isdirected to an inlet of a cyclone air-water separator 34 that isconstructed and arranged to remove moisture and to produce acorresponding dry air mass flow at an outlet of the separator. Theair-water separator 34 may be selected from among commercially availableseparators, for example, the “Cyclonic Mist Eliminator” manufactured byBisco Enterprise, Inc. of Addison, Ill., USA. Any liquid condensate isdumped overboard through a water drain line and exit port 36. Suchcondensate is likely to be produced at altitudes below 20,000 feet wherethe humidity ratio is typically high. The dry air mass flow from theseparator 34 is preferably directed through a desiccant and filter 38arranged at or near the outlet of the separator to lower the relativehumidity (RH) of the air further, and to remove any remainingparticulates and entrained moisture before the air is directed to enterthe equipment bay enclosure 20.

In the embodiment illustrated in FIG. 1, an air circulation fan 40 isincorporated for recirculating air that exits from ports 60 provided inthe equipment bay enclosure 20, in response to a signal from thecontroller 32. When activated, the fan 40 directs warm air that exitsthe enclosure 20 from the ports 60, through a duct 41 and a check valve42. The check valve 42 operates to block the flow of dry air at theoutlet of the separator 34 from entering the duct 41 when the ram airvalve 30 is throttled open. Downstream from the air circulation fan 40,the duct 41 is preferably formed and arranged to open into the outlet ofthe cyclone separator 34 so that the flow of recirculated air in theduct 41 enters the separator outlet at an angle of approximately 15degrees with respect to the direction of flow of the dried air from theseparator 34, thus becoming entrained with the dried air.

An air heater assembly 50 has a heater intake 52 in communication withthe outlet of the separator 34, and a heater exhaust manifold 54 thatopens into the equipment bay enclosure 20. The air heater assembly 50 isconstructed and arranged to heat the dry air mass flow from theseparator 34 to a desired temperature for the payload 26 in response toa signal from the controller 32, and to direct the temperaturecontrolled air flow (the enthalpy) into the thermally insulatedenclosure 20 through the manifold 54. Instead of being disposed at ornear the outlet of the cyclone separator 34, the desiccant 38 may bedisposed in the vicinity of an outlet on the heater exhaust manifold 54.

A relative humidity sensor 55 is disposed in a path of the heated airmass flow from the air heater assembly 50, for example, inside theexhaust manifold 54. The sensor 55 produces an air moisture signal thatcorresponds to the relative humidity of the heated air mass flow.

One or more temperature sensors 56 are disposed inside the equipment bayenclosure 20 in the region of the payload 26, and the sensors 56 operateto produce corresponding air temperature signals. In the embodimentillustrated in FIG. 1, temperature sensors 56 labeled T1 and T2 areplaced at the top and the bottom of the payload 26 (labeled Imager orElectronics in the drawing) to determine the air temperatures above andbelow the payload. A third temperature sensor 56 labeled T3 may beco-located with the humidity sensor 55. If the difference between thetemperatures indicated by sensors T1 and T2 is more than, e.g., 0.5 deg.C., then the openings of the inlet ports 39 on the equipment bay 20 maybe adjusted by operation of the controller 32 until the temperaturedifference is within acceptable limits, typically determined duringlaboratory assembly. Otherwise, during flight, the outputs of thetemperature sensors 56 are averaged by the controller 32. Also, an airpressure sensor 58 is disposed in the equipment bay 20 in the region ofthe payload 26 to produce a corresponding air pressure signal. All ofthe sensors 55, 56 and 58 may be selected from among units that arepresently commercially available.

The signals produced by the air moisture sensor 55, the temperaturesensors 56 and the air pressure sensor 58, are input to the controller32. The controller 32 is configured and arranged to supply the signalsthat activate the ram air control valve 30, the heater assembly 50, theequipment bay inlet ports 39 and the air circulation fan 40, so that thetemperature and the humidity of the air inside the equipment bayenclosure 20 is conditioned according to desired set points that areentered in the controller 32. As mentioned, upon exiting the enclosure20 through the ports 60, conditioned warm air is preferably returned ina closed loop fashion to mix with the dried air from the separator 34.Alternatively, all the air exiting from the ports 60 may be dumpedoverboard. If the pressure of air surrounding the bay enclosure 20reaches a level that prevents outside air from the ram air valve 30 fromentering the enclosure, an exhaust port 61 may be provided on theaircraft frame 14 to vent the surrounding air.

In the FIG. 1 embodiment, during the end of a mission and whiledescending, the ram air valve 30 is preferably kept shut while the aircirculation fan 40 is activated to recirculate air exiting from theequipment bay enclosure 20 with the heater assembly 50 is turned on.This descent mode-of-operation allows the payload 26 to warm-up whileexternal humid air enters the bay as the bay becomes re-pressurized. Inresponse to a signal from the controller 32, the fan 40 directs warm airexiting from the enclosure 20 through the duct 41 and the check valve42. As mentioned, the check valve 42 operates to block the flow of dryair from the outlet of the separator 34 from entering the duct 41 whenthe ram air valve 30 is throttled open.

In an alternate control scheme, during short duration low altitudepenetrations at high Mach number where external aerodynamic boundarylayer air exceeds the air temperature control limit of the internalpayload 26, the ram air valve 30 is temporarily shut and the aircirculation fan 40 is turned ON. In this alternate scheme, air leavingthe equipment bay enclosure 20 is either returned (by entrainment) inclosed loop fashion to mix with the incoming ram air, or the air isdumped overboard.

FIG. 2 is a control diagram of the environmental control system 10 inFIG. 1 including the controller 32. The controller 32 is preferably acommercially available proportional-integral-differential (PID) type ofprocessor or controller, for example, a West 8100 Process Controlavailable from ISE, Inc. of Cleveland, Ohio, USA. Other availablecontrollers or processors may also be used, including a basic ON/OFFtype controller with hysteresis. Separate set points (SP) are entered inthe controller 32 for temperature T (for example, T SP=25 deg. C.+/−1deg. C.); and for relative humidity RH (e.g., RH SP=<40%). The setpoints are arbitrary and preferably can be changed depending on thedesired control.

The temperature and the humidity inside the equipment bay enclosure 20are sensed continuously or at regular intervals over time by thecontroller 32 as required, and the sensed values are subtracted fromtheir respective set points to generate corresponding error signalse(t). In the preferred embodiment, the controller 32 operates in a knownmanner to produce a proportional (P) error term Kp e(t), an integral (I)error term Ki∫_((0→t))e(t)dt, and a differential (D) error term Kdde(t)/dt. As illustrated in FIG. 2, the three error terms for each setpoint are summed (at Σ), and the controller 32 outputs correspondingsignals to the heater assembly 50 and the air control valve 30 to reduceany resulting differences between the temperature and the humiditysensed in the bay enclosure 20 and the corresponding set points.

Those skilled in the art will understand that the proportional gain termKp will effect a change that is proportional to a currently determinederror value. The controller 32 functions as a double processor thatcontrols humidity and temperature simultaneously to ensure that nocondensation will form in the equipment bay enclosure 20. When added tothe proportional term Kp, the integral gain term Ki accelerates themovement of the process toward the set point, and eliminates anyresidual steady state error that may occur when using a proportionalonly type of controller. The differential gain Kd slows the rate ofchange of the controller output, and its effect is most noticeable closeto the controller's set point. Actual values for the gain terms Kp, Kiand Kd may also be determined by persons skilled in the art.

The interaction of the three gain terms as well as the simultaneouscontrol of temperature and humidity is determined by the controller 32.The controller settings for the temperature and humidity are aircraftmode-of-operation dependent. During aircraft ground operation, take-off,climb, mission operation, and descent/landing, the controller 32operates to set the required temperature and humidity conditions for thepayload 26 inside the equipment bay enclosure 20.

An example of an algorithm that may be coded in the controller 32 tothrottle the air control valve 30 and to activate the heater assembly 50to meet the temperature and the humidity set points, is given below.

PID Controller Algorithm set point start temperature relative humidityactual position = temperature actual position = relative humidityprevious error = error or 0 if undefined error = set point − actualposition P = Kp * error I = I + Ki * error * dt D = Kd * (error −previous error) / dt sum = P + I + D If (sum < set point, open valve,close valve) If (sum < set point, increase heat, decrease heat) t + dtgoto start

Example

The following is an example of the operation of the inventive system 10to achieve environmental control between 10 deg. C. and 40 deg. C.inside the bay enclosure 20 over the course of an aircraft mission. Thetable in FIG. 3 illustrates the system operation over those portions ofthe mission between ground operation, take-off, ascent to an altitude of20,000 feet (20 kft) or higher, and descent from 20 kft.

During ground operation, the ram air control valve 30 is opened to allowa ground cooling cart to provide conditioned air into the equipment bay20. With the ground cooling cart connected, the environment inside thebay enclosure 20 is conditioned to a desired temperature and relativehumidity. The air heater assembly 50 is preferably turned ON if the baytemperature sensors 56 indicate a temperature of less than 10 deg. C.,and the heater assembly is turned OFF when the air temperature is above10 deg. C. (or 15 deg. C. if a 5 deg. C. hysteresis is desired).

Also, during ground operation, the flight maintenance operator checks anindicator for the desiccant 38 to ensure that it is blue. If thedesiccant indicator is red, then the desiccant 38 is changed. A typicaldesiccant is “Dessicite 25” available from Engelhard Corporation. Whenthe aircraft is ready to taxi down the runway prior to take-off, theground cooling cart is disconnected. Once the ground cooling cart isdisconnected, the air circulation fan 40 is turned ON and the checkvalve 42 is set open.

As shown in FIG. 3, during take-off and ascent to a nominal altitude of20 kft, the controller 32 operates to keep the ram air valve 30 closedto prevent a large amount of wet air from entering the environmentalcontrol system 10. This action prevents the system 10 from beingover-saturated and ensures longer desiccant life. The air heaterassembly 50 is activated by the controller 32 to obtain the desiredtemperature in the equipment bay enclosure 20, and the air circulationfan 40 is ON forcing the check valve 42 to stay open. Once above 20 kft,the relative humidity in the atmosphere typically becomes negligible.

When the aircraft reaches a nominal altitude of 20 kft, the controller32 is operative to open the ram air valve 30, and to turn the aircirculation fan 40 OFF. In addition, the heater assembly 50 is turned ONto control air temperature inside the equipment bay enclosure 20 to thespecified temperature for the payload 26. The air exiting from theenclosure 20 is either allowed to recirculate by operation of the fan 40and entrainment with the pressurized dry air flow at the outlet of theseparator 34, or the air is dumped overboard when the fan 40 is poweredOFF. If prior to the end of the mission the aircraft dives below 20 kftinto a humid atmosphere before ascending again to a higher altitude,then the ram air valve 30 may remain open and the humid air will bedried by operation of the cyclone separator 34. Any remaining entrainedmoisture in the air (the cyclone separator 34 may be about 90%efficient) is absorbed by the desiccant 38.

Once the mission is over and the aircraft descends below 20 kft, the ramair valve 30 is closed. The fan 40 is turned ON and the airrecirculation loop provides heating to the interior of the equipment bayenclosure 20 to ensure that the air temperature surrounding the payload26 is above an environmental dew point of, e.g., 33 deg. C. (notexceeding 35 deg. C.).

As disclosed herein, the inventive environmental control system 10replaces air cycle and the vapor cycle systems commonly used on aircraftto provide environmental control for sensitive payloads. The system 10creates new value for users by providing the following advantages:

1. Lowering airborne volume consumed for payload thermal management byat least ten cubic feet;

2. Lowering the total aircraft payload weight by at least 80 pounds;

3. Lowering the power draw required from the airborne platform by morethan 2 KW;

4. Increasing the mean time between system failures by a factor of over70;

5. Substantially lowering the cost of a thermal management system forthe payloads;

6. Reducing the time required for life cycle system design, integration,and testing by as much as 40%; and

7. Allowing the user to select most if not all of the major componentsof the system 10 from among commercial off-the-shelf (COTS) products.

While certain embodiments of the invention have been disclosed herein,it will be understood by those skilled in the art that variousmodifications and changes may be made without departing from the spiritand scope of the invention. Accordingly, the invention includes all suchmodifications and changes that lie within the scope of the followingclaims.

1. An environmental control system for a payload equipment bay enclosureon an aircraft, comprising: a ram air scoop arranged to open in aforward direction of the aircraft for obtaining a supply of outside air;an air control valve constructed and arranged for receiving the outsideair supply from the scoop, and for providing a determined air mass flowin response to an air control valve signal; a cyclone air-waterseparator constructed and arranged for receiving the air mass flow fromthe air control valve, and for removing moisture to provide a dry airmass flow from an outlet of the separator; an air heater assemblyconstructed and arranged for heating the dry air mass flow from theseparator to a determined temperature in response to a heater signal,and for directing the heated air mass flow from an outlet of the heaterassembly toward the equipment bay enclosure; an air moisture sensordisposed in a path of the heated air mass flow from the heater assemblyfor producing an air moisture signal corresponding to a relativehumidity of the heated air mass flow; one or more air temperaturesensors disposed in the equipment bay enclosure in the region of apayload when mounted in the enclosure, for producing corresponding airtemperature signals; an air pressure sensor disposed in the equipmentbay enclosure in the region of the payload for producing a signalcorresponding to an altitude of the payload; and a processor orcontroller having inputs coupled to the air moisture sensor, the airtemperature sensors and the air pressure sensor, wherein the controlleris configured and arranged to supply the air control valve signal andthe heater signal according to desired set points for temperature andhumidity for the payload inside the equipment bay enclosure.
 2. Anenvironmental control system according to claim 1, wherein thecontroller is a proportional-integral-differential (PID) type ofcontroller.
 3. An environmental control system according to claim 1,including an air circulation fan and a duct arranged for recirculatingair that exits from the equipment bay enclosure through the duct so thatthe recirculated air mixes with the dry air from the separator inresponse to a signal from the controller.
 4. The system of claim 3,including a check valve arranged in the duct to block the dry air fromthe separator from entering the duct.
 5. An environmental control systemaccording to claim 1, including a desiccant arranged at or near theoutlet of the cyclone separator for lowering the relative humidity ofthe air mass flow from the separator further prior to entering theequipment bay enclosure.
 6. An environmental control system according toclaim 1, including a desiccant arranged in the vicinity of the outlet ofthe air heater assembly for lowering the relative humidity of the heatedair from the heater assembly further prior to entering the equipment bayenclosure.
 7. An environmental control system according to claim 1,including a filter arranged at or near an outlet of the separator forremoving particulates in the air mass flow from the separator.
 8. Anenvironmental control system according to claim 1, including an aircirculation fan arranged for recirculating air exiting from theequipment bay enclosure in response to a signal from the controller. 9.A system according to claim 8, including a duct arranged for receivingthe recirculated air from the air circulation fan, wherein a downstreamend of the duct is formed and arranged to open into the outlet of thecyclone separator so that recirculated air in the duct is entrained withthe air mass flow provided at the outlet of the separator.
 10. A systemaccording to claim 9, wherein the downstream end of the duct is formedso that a flow of recirculated air in the duct enters the outlet of thecyclone separator at an angle of approximately 15 degrees with respectto the direction of the air mass flow provided at the outlet of theseparator.
 11. A system according to claim 9, including a check valvearranged in the vicinity of the downstream end of the duct for blockinga flow of dried air at the outlet of the cyclone separator from enteringthe duct.
 12. An environmental control system according to claim 1,wherein the controller is configured to maintain the air control valvein a closed state from take-off and during ascent to an altitude ofapproximately 20,000 feet during the course of an aircraft missionprofile.
 13. An environmental control system according to claim 8,wherein the controller is configured to maintain the air circulation fanin an on state from take-off and during ascent to an altitude ofapproximately 20,000 feet during the course of an aircraft missionprofile.
 14. An environmental control system according to claim 13,wherein the controller is configured to open the air control valve andto set the air circulation fan to an OFF state at said altitude.
 15. Anenvironmental control system according to claim 13, wherein thecontroller is configured to control the heater assembly at and abovesaid altitude so that the air temperature inside the equipment bayenclosure is controlled according to the desired setpoint fortemperature for the payload.
 16. An environmental control systemaccording to claim 13, wherein the controller is configured to controlthe air control valve at and above said altitude so that the relativehumidity inside the equipment bay enclosure is controlled according tothe desired setpoint for humidity for the payload.
 17. An environmentalcontrol system according to claim 13, wherein the controller isconfigured to control the heater assembly and the air control valve atan altitude of approximately 60,000 feet.